1. Field of the Invention
The present invention generally relates to radar signal processing systems. More particularly, the present invention relates to electronic support systems for detection of enemy radar surveillance and munitions targeting systems. More specifically, the present invention relates to systems for extracting information in the presence of interfering carrier wave signals.
2. Description of the Prior Art
Electronic support (“ES”) systems are used by Navy ships and military aircraft to provide sensitive and timely detection of illuminating signals from enemy radar surveillance and munitions targeting systems, allowing adequate time for the targeted military vessel to successfully engage defensive and offensive counter-measures. ES systems continuously monitor a wide range of radio frequencies (“RF”) for pulsed signals-of-interest (“SOI”) indicative of radar illumination by the enemy. An effective ES capability is required to maximize situational awareness and respond quickly with electronic counter-measures (“ECM”) for force protection when a threat is detected.
The effectiveness of current ES systems is being progressively diminished by increasing levels of RF interference. Current ES systems are based on instantaneous frequency measurement (“IFM”) receiver processing that generally respond to the strongest frequency component in an input received from a single-element antenna or a multiple-element antenna array. Consequently, ES systems may be jammed or severely degraded by high-power in-band interference.
Particularly problematic is interference generated by own-ship RF emitters including on-board satellite communications links. The close proximity of own-ship emitters to the ES antenna array results in high-power jamming of the ES system. Moreover, even with the application of optimal known-waveform cancellation techniques, residual interfering signals may remain and may exceed interference tolerance levels of current ES systems.
The AN/SLQ-32 ES system is a typical ES system in widespread use and is a system that is susceptible to own-ship interference. The AN/SLQ-32 employs an IFM subsystem to generate pulse RF measurements and a direction-finding (“DF”) subsystem to compute pulse direction-of-arrival (“DOA”). Each subsystem performs independent pulse detection operations, which are then associated by time to generate a single pulse descriptor word (“PDW”) containing the combined attributes of time-of-detection (“TOD”), RF, and DOA for each detected pulse. Each merged PDW from these two subsystems is then forwarded as raw data to downstream PDW processing, where “de-interleaving” is performed to identify the individual pulsed emitters. The de-interleaving process accomplishes this function by sorting the PDWs into separate groups based on RF carrier frequency, DOA, TOD, etc. Both IFM and DF subsystems employ analog front-ends to achieve cost-effective broadband coverage for typical AN/SLQ-32 configurations which generally require significant quantities of the front-ends.
Referring to the block representation of FIG. 1, typical IFM subsystems 10 employ a single omni-directional antenna connected to a bank of analog mixer components called IFM discriminators 12. The IFM discriminators 12 are dual-input, dual-output devices where the outputs are provided as amplitude and differential phase measurements of the two inputs 120. An antenna signal is provided at one of the two inputs 120, and a time-delayed version of the antenna signal is presented at the other of the two inputs 120. The magnitude of the delay (also known as the delta-time) is designed to reveal the RF frequency of the dominant signal as a phase term. Generally, between four and eight discriminators 12 are used in a discriminator bank, and a set of time-delays is applied to achieve precision coarse-fine RF frequency measurements.
Interference vulnerability is a consequence of the methods used to process the output of the IFM subsystem. Current IFM processing methods assume that a pulse-of-interest is the dominant signal arriving at an antenna. Until recent years, this assumption was valid for two reasons: First, an illuminating radar signal was expected to be the strongest signal in the input because it must reach the target at high enough amplitude to make a return trip while retaining a detectable signature; second, because radar signals are low duty-cycle pulse trains, only one radar signal will generally be present at any given time, even when multiple radars are illuminating the same target. Hence, conventional IFM processing methods were adequate in the past despite this dominant-signal constraint. In recent years interference levels have increased substantially, to the point that radar signals are frequently weaker than one or several interfering continuous wave (CW, i.e., not pulsed) signals in the same band. These powerful CW interferers include close-by and own-ship communication systems, which are frequently the dominant signal in the IFM system input even when cooperative cancellation techniques are employed to reduce the self-interference levels. Hence, the modern interference environment is now violating the radar dominant signal operating assumption and consequently degrading or disabling IFM system functioning.
Typical DF subsystems deploy 17 directional reception beams fanned uniformly to cover a 90-degree quadrant. DOA measurements are obtained by comparing the received amplitudes in these beams. Because a receiver and processor must be provided for each beam, the beam processing equipment must be small, inexpensive and use low-power. Note, however, that the nature of the directional beam reception pattern facilitates the goals of cost, size and power because the number of interferers falling within the beam is reduced and sensitivity in the look direction is inherently increased. Accordingly, a simple device known as a crystal video receiver (“CVR”) is used at each beam output. The CVR is a single-input, single-output device capable of detecting a strongest signal within the beam and further capable of estimating the amplitude of the strongest signal. However, as is the case with the IFM subsystem, the presence of interference may jam the CVR and causes erroneous amplitude readings that degrade or invalidate DF operation.
Referring again to FIG. 1, currently deployed ES systems are required to monitor wide bandwidths while providing instantaneous response to pulses arriving anywhere in the band. A variety of IFM receivers have been developed that can monitor bands as wide as 16 GHz. Most of these receivers employ a bank of wideband delay line discriminators 12, the outputs of which are passed through a low-pass filter 102 and digitized 104 to generate the so-called digitized video output 14. The video output 14 is then digitally processed to estimate the frequency of an illuminating pulsed SOI. The delay line discriminators 12 often use micro strip technology and coaxial delay lines to achieve the needed hardware reliability and efficiency. Typically, between 4 and 8 discriminators 12 are used in a bank.
A typical product employs logarithmically spaced discriminator delays to compute signal frequency. The shortest discriminator delay τ is selected to yield unambiguous coverage of the desired RF band, where:τ=1/(fmax−fmin).
The phase of this first discriminator traverses one 360-degree revolution across the desired RF range, providing a coarse frequency readout. Typically, the next discriminator in the bank uses twice this delay and traverses two full phase revolutions across the RF range, providing a finer resolution measurement. The third discriminator uses four times the basic delay and traverses four full phase revolutions across the range, and so on to increasingly finer resolution. Other products employ the “binary word” approach, in which linearly spaced delays are used to derive a binary frequency word directly from the bank outputs using voltage comparators. Both of these conventional approaches analyze the angle of the discriminator output, and are therefore subject to two major limitations: performance is optimal only if the SOI pulses are the strongest signal in the antenna; and, approximate low-pass and high-pass filtering techniques known respectively as AC coupling and DC coupling must be used to separate SOI pulse components from carrier wave (“CW”) interference components in the IFM bank video output. However, these products frequently fail to provide the needed discrimination between the different types of signals.
The loss of performance with decreasing signal-to-interference ratio (“SIR”) is an obvious drawback because it makes the ESM system susceptible to simple CW jamming or capture by other incidental, strong signals. But the use of AC/DC coupling separate pulse and CW components creates system vulnerabilities whether or not jamming is in use, and introduces performance degradation due to reasons including: the inability to separate pulsed and CW emitters unless the CW emitters maintain constant amplitudes; and ineffective separation of pulsed and CW emitters as pulse duration increases.
A common approach to mitigate these drawbacks is to deploy notch filters and to remove the CW interferers before they reach the IFM discriminator bank. While these approaches provide limited mitigation of the problems when a small number of large-amplitude CW interferers are present, they usually require onerous manual operation and they often present other problems. For example, the need to cover wide bandwidth necessitates the use of analog filters which limit the ability to generate narrow notches using even the best of current analog technologies. Excessive notch width distorts the pulsed signal-of-interest (SOI) and limits the number of notches that can be deployed without severely degrading the required pulse measurements.
Therefore, what is needed is a method and system for retrofitting IFM-based ES systems that handles interference environments, and improves the threat pulse detection and measurement functions over a broad range of interference and noise conditions.